<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
    <style>
      * {
        margin: 0;
        padding: 0;
      }
      ul {
        display: flex;
        border-bottom: 2px solid #e01222;
        padding: 0 10px;
      }
      li {
        width: 100px;
        height: 50px;
        line-height: 50px;
        list-style: none;
        text-align: center;
      }
      li a {
        display: block;
        text-decoration: none;
        font-weight: bold;
        color: #333333;
      }
      li a.active {
        background-color: #e01222;
        color: #fff;
      }
    </style>
  </head>
  <body>
    <div id="app">
      <ul>
        <li v-for="(v, i) of list" :key="v.id">
          <!-- 当【激活的索引】和【当前索引】一样，就为true，就有这个类名 -->
          <a
            :class="{ active: activeIndex === i }"
            href="#"
            @click="activeIndex = i"
            >{{ v.name }}</a
          >
        </li>
      </ul>
    </div>
    <script src="./vue.js"></script>
    <script>
      const app = new Vue({
        el: '#app',
        data: {
          activeIndex: 0,
          list: [
            { id: 1, name: '京东秒杀' },
            { id: 2, name: '每日特价' },
            { id: 3, name: '品类秒杀' },
          ],
        },
      });
    </script>
  </body>
</html>
